Abstract: A radioimmunoassay which comprises the steps of reacting a first antibody (A) on an insoluble solid, which is specific for an antigen (B); an antigen (B); and a second antibody (C) derived from an animal different from that of the first antibody (A), which binds to the antigen (B) under conditions that the concentration of the second antibody (C) is higher than the dissociation constant between the second antibody (C) and the antigen (B) to obtain an immune complex (D); reacting the immune complex (D) with a radiolabeled probe (E) and then measuring the radioactivity of the solid precipitate or the reaction mixture, to measure the quantity of antigen (B). The method provides an assay with high sensitivity which does not depend on the sensitivity of the second antibody to be used.